MemCpyOptimizer: Use max legal int size instead of pointer size

If there are no legal integers, assume 1 byte.

This makes more sense than using the pointer size as
a guess for the maximum GPR width.

It is conceivable to want to use some 64-bit pointers
on a target where 64-bit integers aren't legal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190817 91177308-0d34-0410-b5e6-96231b3b80d8